In recent years, sensor network systems which comprise a sensor node and a sensor network management server are being developed. The sensor node observes a state (sensing data) of a man or a point etc., and transmits the observed sensing data to a sensor network management server. The sensor network management server performs various processing based on the received sensing data.
For the sensing data, only the latest values may be used, or history data at a previous fixed time may be used. For this purpose, techniques have been disclosed for managing the latest sensing data and historical sensing data (for example, refer to JP-A No. 2004-247785 and JP-A No. 2004-46780).
According to JP-A No. 2004-247785, a substation stores data with a time stamp with a predetermined frequency. A main station connects a circuit to the substation, and collects data stored by the substation. The main station determines whether there is any data missing based on a time stamp. When it determines that some data is missing, the main station collects the missing data from the substation by transmitting a time entry to the substation.
According to JP-A No. 2004-46780, a plant server holds stored information and a plant controller collects real-time information. A center server transmits either the stored information or real-time information to an end user terminal according to a request from an end user terminal.